The invention concerns an NMR spectrometer with an NMR magnet system disposed in the helium tank of a cryostat, and with an NMR probe head disposed in a room temperature bore of the cryostat which contains a cooled RF resonator for receiving NMR signals from a sample to be examined, and with a cooled pre-amplifier, wherein the NMR probe head is cooled by a common multi-stage compressor-operated refrigerator, the refrigerator comprising a cold head and several heat exchangers at different temperature levels, wherein the refrigerator is disposed at a spatial separation from the cryostat in a separate, evacuated and thermally insulated housing, and wherein at least one cooling circuit is provided comprising cooling lines, which are thermally insulated by a transfer line, extending between the housing containing the heat exchangers and the NMR probe head.
A device of this type is disclosed in U.S. Pat. No. 5,889,456.
The NMR probe head of an NMR spectrometer is located together with a measuring device in the bore of a magnet cryostat. This magnet cryostat houses a superconducting coil which generates the magnetic field required for the NMR measurements. The NMR probe head as well as the magnet cryostat must be kept at very low temperatures during operation. The thermal loss generated through thermal conduction and thermal radiation is therefore a problem.
Two important fields of application of cryocooling systems in the field of NMR are therefore cooling of the cryo probe heads for cryogenic cooling of the RF resonator and of the pre-amplifier and for cooling the superconducting NMR magnets for cryogenic cooling of the cryostat and therefore realization of a zero evaporation rate both for LN2 and LHe (LN2=liquid nitrogen, LHe=liquid helium).
Different systems from different companies are on the market today which are designed to solve this problem. The company JASTEC (Japan) [1] provides a cooling system for cooling the LN2 region of a cryostat for superconducting magnet systems up to 400 MHz. It contains a low-vibration pulse tube cooling unit which, however, has a maximum cooling power of less than that of conventional GM cooling units (GM=Gifford-McMahon). A zero evaporation rate for LN2 is therefore achieved only for NMR magnet systems of up to at most 400 MHz.
The company NIHON THERMAL (Japan) [2] provides a cooling system for cooling the LN2 region of a cryostat for superconducting magnets up to 600 MHz. It contains a powerful GM unit which generates stronger vibrations than that of JASTEC but provides a zero evaporation rate for LN2 at 600 MHz magnet systems.
There are conventional superconducting NMR magnets made by the company OXFORD INSTRUMENTS SUPERCONDUCTIVITY (GB) [3], which comprise a cooling system for cooling the LH2 and the LHe regions of the cryostat. The cooling system utilizes a low-vibration pulse tube cooling unit which is directly mounted to the cryostat and achieves a zero evaporation rate both for LN2 and for LHe.
The company Bruker BioSpin AG [4] and the company VARIAN (USA) [5] each distribute a cooling system called “CryoPlatform” (Bruker BioSpin) and “Cryo Bay” (Varian) for cryogenic cooling of RF resonators. These two cooling systems contain a GM cooling unit and use cold He gas as transport and cooling means.
U.S. Pat. No. 5,889,456 discloses a device comprising a refrigerator for cooling the NMR probe head. Heat exchangers and a transfer line from the refrigerator to the NMR probe head transfer the cooling power produced by the refrigerator. The NMR probe head is supplied with coolant by pumps or compressors via the transfer lines. The cooled components of the probe head are usually at temperatures of 10–60 Kelvin. A Gifford-MacMahon cooler (GM) or a pulse tube cooler (PT) are usually used as refrigerators.
In the majority of NMR magnet systems without active cryocooling, the holding time for LHe is more than 6 months, however, only two to three weeks for LN2. The short holding time for LN2 is due to cryostat construction only and applies as long as no excessive expense is incurred for thermal shielding of the LN2 tank.
The LN2 loss is on the order of magnitude of 10–20 Watt at approximately 77K which could be easily compensated for with a small active cooling unit. Small units with low power also require different basic units such as e.g. compressors, and are therefore not inexpensive. The expense required to solve only the object of reducing or completely compensating for the LN2 loss may therefore often be too high.
It is therefore the underlying purpose of the invention to propose an NMR spectrometer which permits matching of the holding time of LN2 to that of LHe without great expense to reduce the service costs associated with refilling the cryogenic liquids.